This invention relates to polymer bound stabilizers and more particularly to combinations of polymer bound UV stabilizers and/or polymer bound antioxidants, preferably where the stabilizing groups provide a synergistic effect. This invention also relates to the use of these polymer bound stabilizers to stabilize polymers or polymer blends against thermal and/or photooxidative degradation.
When exposed to sunlight or to strong fluorescent illumination, most plastics undergo degradation. This usually results in color development and loss of physical properties. To overcome these problems polymers are normally protected against photochemical attack by the incorporation of ultraviolet stabilizers, hindered amine light stabilizers, ultraviolet quenchers, peroxide decomposers, free radical chain breakers, or combinations of the above.
Three factors affect the performance of the stabilizer in a polymer composition: the intrinsic activity of the stabilizer functional group on a molar basis, the compatibility or solubility of the stabilizer in the polymer system, and the ability of the stabilizer to remain in the polymer system. The third factor is often the dominant factor (J. K. Kuczkowski, J. G. Gillick, Ruber Chemistry and Technology, 57, pp 621-651 (1984); G. Scott, New Development in Rubber-bound Antioxidants, Rubbercon 77, Int. Rubber Conf., 1977, 1, paper #19). Consequently, there has been a considerable amount of effort in the development of stabilizers that are less volatile, more compatible and less readily lost during fabrication and exposure to the environment. Engineering thermoplastics are processed at high temperatures so it is essential to use high molecular weight stabilizers that are not lost through drying, extrusion and molding steps. For polymers that come in contact with foodstuffs it is important that the stabilizers be non-toxic or non-extractable from the polymer into the foodstuff. Obviously, polymer bound stabilizers are preferred where FDA approval is required in the end-use.
Various approaches have been used to overcome volatility and compatibility shortcomings. For example, stearyl and lauryl diesters of thiodipropionic acid are commonly employed to overcome the volatility and odor problem of the lower molecular weight esters. Tetraesters of pentaerythritol, such as Irganox 1010 (product of Ciba Geigy) are also extensively used to minimize volatility problems. Multifunctional stabilizers have been prepared by reacting one type of stabilizer with another to obtain a higher molecular weight compound having dual functionality or by reacting two or more stabilizers with a multifunctional coupling agent (e.g., cyanuric chloride) in a stepwise fashion (U.S. Pat. No. 3,257,354).
Another approach to solving the volatility and migration problems of the stabilizers has been to prepare stabilizers with polymerizable groups and then either polymerize the monomeric stabilizers to homopolymers or copolymerize the stabilizer with the monomer of the polymer requiring stabilization. (J. Fertig, A. I. Godberg, M. Shoultchi, J. Appl. Polym. Sci., 10, pp 672 (1966); G. Scott, Developments in Polymer Stabilization, Vol 4, G. Scott, Ed., App. Sci. Pub., London, 1981, pp 181-221).
The more popular approach is to copolymerize the polymerizable stabilizer with another monomer. There are numerous examples of copolymerizable antioxidants and copolymerizable UV stabilizers found in the literature (F. E. Karrer, Makromol. Chem., 181, pp 595-633 (1980); J. K. Kuczkowski, J. G. Gillick, Rubber Chemistry and Technology, 57, pp 621-651 (1984); D. Bailey, D. Vogl, J. Macromol. Sci. Rev. Macromol. Chem., C14(2), pp 267-293 (1976)).
Six examples of polymers or copolymers with multiple bound UV stabilizers and antioxidants have been found. In the first (W. Dickstein, D. Vogl, J. Macromol. Sci. -Chem., A22(4), pp 387-402 (1985)), copolymerizations and terpolymerizations with a combination of a polymerizable N-phenylbenzotriazole and a polymerizable hindered phenol were carried out with methacrylate esters, N-vinylpyrrolidone, and 1,1,1-tris(trimethylsiloxy)methacrylatopropylsilane. The copolymerizable UV stabilizers were 2-(2-hydroxy-5-isopropenylphenyl)-2H-benzotriazole and 2-(2-hydroxy-5-vinylphenyl)-2H-benzotriazole and the copolymerizable antioxidant was 4-isopropenyl-2,6-di-t-butylphenol.
The second example (U.S. Pat. No. 4,523,008) involves a mixture of the same copolymerizable UV stabilizers and copolymerizable antioxidants which were copolymerized with an unsaturated polyester and styrene.
The third example (U.S. Pat. No. 4,276,401) describes polymer bound multifunctional light stabilizers prepared by copolymerizing N-substituted acrylamide or methacrylamide monomers. One of the monomers contained a hindered amine substituent and the other monomer contained a light stabilizing substituent.
In the fourth example (G. Scott, New Developments in Rubber-Bound Antioxidants, Rubbercon 77, Int. Rubber Conf., 1977, 1, paper #19), antioxidants and UV stabilizers containing thiol groups were attached to rubber modified thermoplastics such as ABS or other rubber latices using peroxide initiators.
In the fifth example of multiple polymer bound stabilizers, Borg-Warner patented the method of attaching the above mentioned thiol antioxidants and thiol UV stabilizers to rubber modified thermoplastics in the presence of a peroxide initiator in a melt processing step (European Patent Appln. 84,882; CA99: 141092n).
There has been a considerable amount of activity in the preparation of polymer bound stabilizers by modifying copolymers containing reactive functionalities with stabilizers containing groups that react with the reactive functionality of the copolymer (the sixth example). An example of such a modification is the transesterification of ethylene-ethyl acrylate copolymers with 2,2,6,6-tetramethylpiperidin-4-ol (U.S. Pat. No. 4,413,096). Japanese Patent No. 69/02,719 (CA70: 1071524y) is the only example where a functionalized copolymer is reacted with a mixture of reactive (glycidyl substituted) stabilizers (sulfide and hindered phenol antioxidants) to provide a synergistic system.